Showing papers on "Pearlite published in 2017"

Open-source wire and arc additive manufacturing system: formability, microstructures, and mechanical properties

Abstract: The inexpensive cost and high manufacture efficient metal wire and arc additive manufacturing (WAAM) system was designed in this work. Its application potential was evaluated from the three aspects of formability, microstructures, and mechanical properties. By using compulsory cooling solution implemented in the open-source WAAM system, the complex-shaped metal parts were deposited completely with no obvious defects, such as cracks, pores, or incomplete fusion. The properties of the WAAM part were evaluated by optical microscopy (OM), scanning electron microscopy (SEM), microhardness, and microtensile test. The results indicate that the formability of metal parts fabricated by the open-source WAAM system was improved by using compulsory cooling solution. The microstructures of the WAAM part are exhibited as granular structure which consisted of the granular ferrite and the residual austenite interspersed with a little pearlite in the intermediate zone. And the average ferrite grain size of non-overlapping layer is relatively smaller than that of overlapping layer. The specimen perpendicular to the building direction exhibits a better mechanical property.

Structural and Microhardness Changes After Turning of the AISI 1045 Steel for Minimum Quantity Cooling Lubrication

Abstract: This work presents the cooling effect under minimum quantity cooling lubrication and dry cutting on structural changes and microhardness of the ferritic-pearlitic AISI 1045 steel after turning. Due to the fact that the AISI 1045 steel has a two-phase structure, microhardness tests using the Vickers method were conducted with a load of 0.05 HV separately for ferrite and pearlite grains. The tests showed that cooling of the cutting zone under minimum quantity cooling lubrication (MQCL) condition decreased the depth of the hardened layer compared to dry cutting by approximately 40% for both pearlite and ferrite. Scanning electron microscopy analysis revealed that applying MQCL limits the formation of plastic deformations, among others, by reducing the surface crumple zone by approximately 50% compared to dry cutting. As a result of cooling being applied to the cutting zone using the MQCL method, the average diameter of ferrite grains has been decreased in the entire surface area compared to dry cutting. When using dry cutting, clear structural changes of the surface layer were also observed. This may be the result of sulfide inclusions which have formed, causing microcracks on the edge of the hardened layer and surface deformation visible as notches.

Local cementite cracking induced by heterogeneous plastic deformation in lamellar pearlite

Abstract: To fully understand the fracture mechanism of pearlitic steel, the effects of lamellar alignment on both cementite cracking and plastic deformation behavior were investigated in two pearlitic materials with lamellar and spheroidized cementite phases. Digital image correlation revealed that local strain develops heterogeneously in the lamellar pearlite structure, but homogeneously in the spheroidized structure. The heterogeneous local strain distribution tends to coincide better with a pearlite colony than with a pearlite block. This finding suggests that the plastic deformation behavior of pearlite is strongly affected by the alignment of the ferrite/cementite lamellae. A detailed crystallography-based analysis revealed that the ferrite matrix was significantly plastically deformed in colonies in which the lamellae were aligned at approximately 45° relative to the direction of applied tension, regardless of the limited deformation of the cementite phase; this caused high degrees of strain in such colonies. In contrast, plastic deformation of the ferrite matrix was restricted by the lamellar cementite in colonies in which the lamellae were aligned parallel to the tensile direction. As a result, the lamellar cementite experienced plastic deformation simultaneously with the ferrite matrix as the applied strain was increased. This simultaneous plastic deformation caused shear deformation in lamellar structures on identical slip systems, by which the ductile fracture associated with cementite cracking occurred.

Effect of boron concentration on microstructures and properties of Fe–B–C alloy steel

Abstract: The solidification microstructure, types of eutectic borocarbides, heat treatment properties and wear resistance of steel with x wt% B–0.4 wt% C–6.0 wt% Cr–4.0 wt% Mo–1.0 wt% Al–1.0 wt% Si–1.0 wt% V–0.5 wt% Mn (x = 1.0, 2.0, 3.0) have been investigated in this present study. The results indicate that the as-cast Fe–B–C alloy steel consists of pearlite, ferrite, and borocarbides M2(B,C) (M = Fe, Cr, Mo, V, Mn). After quenching or quenching and tempering treatment, ferrite and pearlite transform into martensite. With the increase of boron content, the macrohardness of alloys increases obviously while wear loss decreases. Borocarbides with chromium addition have good toughness and no cracks are observed on worn surfaces. The wear mechanism changes from micro-cutting accompanied with the spalling of borocarbides to single micro-cutting with the boron content rising.

Thermo-mechanical fatigue property and life prediction of vermicular graphite iron

Abstract: Thermo-mechanical fatigue (TMF) failure is the major problem of the cylinder head subjected to combined variations in temperature and loading during operation. This study mainly focuses on the TMF property and the life prediction of vermicular graphite iron (VGI). The ferrite clusters can be easily found from the microstructure and fractography images. Compared with the TMF experimental data testing at 125–400 ℃ and 125–500 ℃, significant cyclic hardening occurs in the former and slight hardening does in the latter. Depending on the difference in the damage mechanism between TMF and iso-thermal low-cycle fatigue (LCF), based on the hysteresis energy, a life prediction method has been proposed at first. By the minimum amount of LCF and TMF tests, the present method can predict the TMF life rapidly, accurately and cheaply. And based on the difference in the fatigue crack propagation thresholds between pearlite and ferrite, the fracture mechanism of TMF was also discussed.

Structure-properties relationship of ultra-fine grained V-microalloyed dual phase steels

Abstract: The effect of vanadium microalloying on ultra-high strength dual phase (DP) ferrite-martensite steel microstructure and properties was studied. It was found that the addition of 0.14 wt% V to a Fe-0.18C-1.5Mn-0.3Si-0.008N reference alloy introduced very significant ferrite grain size refinement in the cold rolled and annealed state. During continuous annealing the initial ferrite to austenite transformation kinetics were strongly retarded, however under slow cooling both pearlite and bainite transformations were suppressed indicating increased hardenability. After cold rolling and intercritical annealing at 750 ⁰C intense V(C,N) precipitates (mean radius 3.7 nm) were observed in the ferrite phase whereas precipitates were scarce in martensite (austenite) and much larger (mean radius 6.7 nm). Significant gains in YS, UTS and work hardening rate were observed at low martensite fractions due to a combination of selective precipitation strengthening and grain refinement of ferrite. However, at higher martensite fractions (> 45%) the YS, UTS and work hardening rate became lower than the reference, primarily due to softening of the martensite. The latter was attributed to the fixing of solute carbon by V(C,N). The net increase in tensile strength with martensite content of the vanadium alloy was ~ 4 MPa/%α’ compared to ~ 16 MPa/%α’ for the reference alloy. A recently developed size-sensitive mean field structure-properties model was extended to capture these microalloying effects. At iso-tensile strength both the fracture strain and hole expansion behaviour of the new microalloyed steel showed improved performance over the reference.

Effect of interlamellar spacing on fracture toughness of nano-structured pearlite

Abstract: Microstructural refinement to the nanoscale imparts tremendous strength to metals while following the Hall-Petch relationship. However, most manufacturing methods to make nanostructured metals are limited by the maximum size of the final product. Extremely fine pearlitic steels with mean true inter-lamellar spacing of 110 nm and 63 nm while having yield strength of 0.79 GPa and 1.08 GPa respectively have been produced by suitable alloying and appropriate undercooling to enhance the free energy available for transformation. The stronger pearlite shows enhanced plane strain fracture toughness thus offering a combination of high damage resistance and tolerance.

Towards the ab initio based theory of phase transformations in iron and steel

Abstract: Despite of the appearance of numerous new materials, the iron based alloys and steels continue to play an essential role in modern technology. The properties of a steel are determined by its structural state (ferrite, cementite, pearlite, bainite, martensite, and their combination) that is formed under thermal treatment as a result of the shear lattice reconstruction γ (fcc) → α (bcc) and carbon diffusion redistribution. We present a review on a recent progress in the development of a quantitative theory of the phase transformations and microstructure formation in steel that is based on an ab initio parameterization of the Ginzburg–Landau free energy functional. The results of computer modeling describe the regular change of transformation scenario under cooling from ferritic (nucleation and diffusion-controlled growth of the α phase) to martensitic (the shear lattice instability γ → α). It has been shown that the increase in short-range magnetic order with decreasing the temperature plays a key role in the change of transformation scenarios. Phase-field modeling in the framework of a discussed approach demonstrates the typical transformation patterns.

Evolution of Microstructure from the Surface to the Interior of Cr-Mo Steel by Water Jet Peening

Abstract: The microstructure and hardness on and below the surface of Cr-Mo steel (SCM435) treated by water jet peening (WJP) were investigated using scanning electron microscopy and micro Vickers hardness measurements. The change of the surface residual stress caused by the WJP treatment influenced the surface microstructure and surface hardness of the SCM435 steel. Cementite in the pearlite phase tended to protrude as the duration of WJP was increased. Voids were formed in the area 0.5 - 1.0 mm below the surface and also at grain boundaries between ferrite and pearlite grains, whereas no voids were formed in the depth range from 2.0 to 3.0 mm below the surface.

Effects of niobium addition on microstructure and tensile behavior of as-cast ductile iron

Abstract: The effects of niobium addition up to 0.11 wt% on the microstructure and tensile properties of as-cast ductile iron (ACDI) were investigated. Metallographic analyses by both optical microscopy (OM) and scanning electron microscopy (SEM) indicated that niobium (Nb) promoted the formation of pearlite, reduced pearlite lamellar spacing and decreased the extent of graphitization taking place in the Nb-alloyed ACDI. The nodularity and nodule counts of graphite changed insignificantly when the Nb content was less than 0.08 wt% in the ACDI. The analysis of precipitates by transmission electron microscopy (TEM) revealed that nano and micro sized (Nb, Ti)C carbides acted as nucleation site for graphites, and promoted the formation of large graphite nodules with low roundnesses as Nb content rose above 0.08 wt%. The results of tensile testing showed that the yield strength, ultimate tensile strength and elongation of the ACDI with 0.08 wt% Nb increased by 12.1%, 11.2% and 14.3% over those of the Nb-free ACDI, respectively. The optimum values of the yield strength, tensile strength and elongation of the Nb-alloyed ACDI were found to be 418 MPa, 746.0 MPa and 8.0%, respectively, at the Nb content of 0.08 wt%. The high strain hardening rates of the Nb-containing ACDIs implied that they were capable of spontaneously strengthening itself increasingly to a large extent, in response to a slight plastic deformation after yielding.

Effect of microstructure evolution on anisotropic fracture behaviors of cold drawing pearlitic steels

Abstract: The effect of microstructure evolution on the anisotropic fracture behaviors of cold drawing pearlitic steels was investigated by conducting an in situ tension test with scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electro backscatter diffraction (EBSD). Results revealed three patterns of crack propagation in the pearlitic microstructure: shear cracking, cracking along the pearlite colony interface, and cracking with respect to pearlite phase boundary. The crack propagation mode is mainly determined by shear cracking as strain increases because of the successive microstructure evolution. After cold drawing is obtained from the strain of e=0 to e=0.8, the crack propagation path became deflected with the anisotropic fracture behavior. As the strain increases to 1.6, the degree of deflection decreases. Combined with the main function of shear cracking in crack propagation, the gradual formation of ferrite texture causing the variation of grain orientation distribution with respect to tensile axis also affects the anisotropic fracture behavior.

Effects on hardness and microstructure of AISI 1020 low-carbon steel processed by high-pressure torsion

Abstract: Low-carbon steel AISI 1020 was subjected to high-pressure torsion (HPT) with 6.0 GPa pressure through 1/4–5 turns. The microstructures of the samples in each turn were studied by means of X-ray diffraction (XRD) analyzing the changes in micro-strain, crystallite size and lattice parameter. Vickers testing was utilized to study the microhardness behavior of the samples subjected to HPT processing. The morphology evolution of the samples and especially the changes in ferrite and pearlite structures were studied for different numbers of turns using scanning electron microscopy (SEM).

Effects of coiling temperature and pipe-forming strain on yield strength variation after ERW pipe forming of API X70 and X80 linepipe steels

Abstract: Since pipes are undergone repeated tension and compression strains during pipe-forming, and flattening, flattened sheets often show too higher or lower yield strength than hot-rolled coils, which poses to difficulties in satisfying yield strength standards. In this study, effects of microstructure and pipe-forming strain (thickness/diameter (t/D)) on yield strength variation were investigated in X70 (483 MPa) and X80 (552 MPa) linepipe steels fabricated by controlling Mo content and coiling temperature, and their yield strength, strain hardening exponent, and Bauschinger stress parameter were measured by tension-compression tests with varying tensile-pre-strain. In the X80 steels whose Mo content was higher than that of the X70 steels, the higher Mo content promoted the formation of low-temperature transformed microstructures such as acicular ferrite (AF), granular bainite (GB), bainitic ferrite (BF), and martensite-austenite (MA) constituent, which played a role in decreasing Bauschinger effect. The reduction in yield strength was smaller in the X80 steel than in the X70 steel. As the coiling temperature decreased, the volume fractions of AF, BF, and pearlite increased, while those of QPF, GB, and MA decreased, and led to the increase in yield strength by about 30 MPa. The yield strength slightly increased after the pipe forming at higher coiling temperature, while it was largely reduced at lower coiling temperature. When the steels having different t/D were compared, the yield strength after the pipe forming increased largely by 65 MPa under the higher t/D as the strain hardening effect overrode the Bauschinger effect. In order to prevent or minimize the large reduction in yield strength after the pipe forming, low-temperature transformation microstructures, coarse grain size, and high t/D were desirable.

Transformations of Supercooled Austenite in a Promising High-Strength Steel Grade Under Continuous Cooling Conditions

Abstract: Special features of the transformations of supercooled austenite occurring under continuous cooling of a promising high-strength steel grade not standardized in the Russian Federation are determined. A method for evaluating the volume fractions of structure constituents formed in the steel as a result of cooling from 925°C at various constant rates within 0.025 – 75 K/sec is proposed and tested. The results are generalized in the form of a thermokinetic diagram of transformations of supercooled austenite.

Microstructures and Mechanical Properties of as-Drawn and Laboratory Annealed Pearlitic Steel Wires

Abstract: Near eutectoid fully pearlitic wire rod (5.5 mm diameter) was taken through six stages of wire drawing (drawing strains of 0 to 2.47). The as-drawn (AD) wires were further laboratory annealed (LA) to re-austenitize and reform the pearlite. AD and LA grades, for respective wire diameters, had similar pearlite microstructure: interlamellar spacing (λ) and pearlite alignment with the wire axis. However, LA grade had lower hardness (for both phases) and slightly lower fiber texture and residual stresses in ferrite. Surprisingly, essentially identical tensile yield strengths in AD and LA wires, measured at equivalent spacing, were found. The work hardened AD had, as expected, higher torsional yield strengths and lower tensile and torsional ductilities than LA. In both wires, stronger pearlite alignment gave significantly increased torsional ductility.

Study on microstructure and mechanical properties of dissimilar steel joint developed using friction stir welding

Abstract: In the present study, microstructure and mechanical properties of dissimilar weld of structural steel and ferritic stainless steel (FSS) plates of thickness 3 mm were investigated. The plates were butt welded by friction stir welding and defect-free welds were produced at a traverse speed of 20 mm/min and rotational speed of 508 rpm using a tungsten carbide tool. The weld joint consisted of alternate bands of both steels resembling an onion ring pattern. In the weld joint, six distinct regions were found including both the base metals. The stir zone of structural steel revealed refined grain structure of ferrite, pearlite, and martensite whereas in ferritic stainless side, highly refined ferritic grains with grain boundary martensite was observed and also confirmed by x-ray diffraction (XRD). The hardness of the weld joint varies from 186 to 572 HV. This scatter of hardness in stir zone is due to the presence of metal from both sides. The ultimate tensile and yield strengths of the transverse weld specimens was higher than the structural steel base metal whereas lower than the ferritic stainless steel, having fracture from structural steel side.

Microstructure and properties of low carbon steel after surface alloying induced by high current pulsed electron beam

Abstract: To improve the surface properties of low carbon steels, Cr alloying layer was introduced on the steel substrate irradiated by high current pulsed electron beam (HCPEB) alloying treatment. The microstructures of the Cr alloying layer were studied by means of XRD, OM, SEM and TEM. The properties of microhardness and corrosion resistance were also tested. Microstructure observations revealed that after HCPEB alloying, the surface was remelted and the thickness of the remelted layer was about 4–6 μm. The content of Cr in the remelted layer was higher than that of the matrix. TEM results revealed that the cementite in pearlite was dissolved, and a high density of dislocations and tiny reinforcing phases were formed in the alloying layer. Both of the microhardness and corrosion resistance of the irradiated samples were significantly improved after HCPEB alloying treatment.